here's an idea...

This worked great for me, I took egg crate and cut it just smaller that the back wall of the tank. I then attached the cork bark from the back with drywall screws and washers. I filled the back with Great Stuff,(expanding foam insulation), when it dried, I took a saw and cut off the foam even with the egg crate. This gave me a one piece corkbark background for my 45 gallon tank with all of the voids filled. I just put a few small dabs of silicone to attach it to the glass. This was much easier than trying to attach each piece of bark individually and filling in the gaps. It also comes out easy incase you need to breakdown the tank. It's been two years and the foam is still fine, no need to seal or waterproof. The only thing I would do differently is use those disposable latex gloves,I think it took a month to get it off my hands.

Tom

:I too have thought of this. I'm thinking about using that expanding insulation foam (comes in aerosol cans at Home Depot/Lowes). This is my plan on how to do it:

:I'd build a sort of box without a top measuring about 1" tall x the width of the cork slab x the length of the slab. Then I'd drill a hole in one end of the "box" to squirt the foam into and a small hole at the other end to allow for any over-fill as the foam expands. then line the inside of the box with something like tin foil (I'm not sure on this part, need to do some testing to find out what this foam sticks to and what wont stick to). then I'd firmly clamp the slab over the open box, and squirt the stuff into the inlet hole until it began seeping out the expansion hole at the other end. let it dry, then unclamp, and you should have a perfectly flat back to your cork now. I figure you'd probably have to seal the foam at this point, and I'm still a little unsure about what to use for sealant. Once it's sealed however, it could be siliconed into the viv with no chance of animals getting behind it!